Graphs

This minor release includes:
* Sparse Manifold Clustering and Embedding, available as `graphs.construction.smce_graph`.
* Bugfix for `locality_preserving_projections` when using a sparse Laplacian matrix.
* Improved keyword arguments for some methods.

This minor release includes:
- a new construction method: `graphs.construction.urquhart_graph()`
- a `weighted` option for `graphs.construction.b_matching()`
- a `metric` option for `graphs.construction.neighbor_graph()`/`nearest_neighbors()`, and a soft deprecation of the `precomputed` kwarg
- smarter dispatching logic and more kwargs for the `Graph.shortest_path()` method
- documentation fixes
- more consistent use of warnings and errors
- tweaks to the default graph visualization style

This is a bugfix-only release, without any new features.

This release only includes 14 commits, but it restructures a lot of code and made some big breaking API changes.

Big changes:
- `foo.matrix(dense=True, csr=True)` is now written as `foo.matrix('dense', 'csr')`. This affects a lot of code, but it needed to be fixed.
- The dataset code from the `graphs.generators` module has been split out into a new module, `graphs.datasets`.
- The `random_graph` function has been moved from `graphs.construction` to `graphs.generators`.
- Two new functions for creating graphs have been added: `chain_graph` and `lattice_graph`.
- The defaults for `foo.plot()` are now `directed=False` and `weighted=False`, because that's what most of the use-cases are.
- The Graph object has two new methods: `bicolor_spectral` and `isograph`.

This release covers 62 commits, and contains many improvements and bug fixes:
- Added a `TransformMixin`, which provides many convenient methods for transforming graphs.
- `Graph.pairs` and `Graph.edge_weights` now support a `directed` kwarg.
- Added a `Graph.subgraph` method, for extracting a subgraph as a separate `Graph` object.
- Added a `Graph.remove_edges` method.
- `Graph.from_edge_pairs` now supports the case where weights are provided for symmetric pairs.
- Added a benchmark for neighbor graph construction.
- Added an example script which demonstrates nine methods for building a graph on the swiss roll toy dataset.
- Improved docstrings for methods overridden by `Graph` subclasses.
- Refactored the Cython code for easier distribution and use.

... and much more! The API is still evolving, but most of the changes should be backwards-compatible with v0.0.3.